This invention relates to a flow machine and more specifically it relates to a centrifugal pump having a unique hydraulic characteristic that the pump spontaneously establishes a circulating flow through the pump housing when pump flow is reduced to the level that pump cooling is needed. The present invention also relates to energy saving at low pumping capacities in which the hydraulic energy of the circulating flow is recovered by a turbine action, which is otherwise wasted in a conventional pumping system.
Protecting pumps from overheating at shutoff and at very low capacities is one of the most common and troublesome problems concerning centrifugal pump applications. A thermodynamic phenomenon is that a centrifugal pump always converts a part of the mechanical power into heat when running. At very low capacities most shaft power consumed is converted into wasteful heat in the pump casing, which soon becomes excessive to the heat removal capability of the through-flow. The unbalanced heat will raise the liquid temperature in the pump and eventually cause pump component damage and plant operation problems.
Operating centrifugal pumps at shutoff and near shutoff is frequently required in pump installations. For example, in a boiler feed pump installation the demanded flow capacity may vary from zero to full pump capacity depending upon the load on the turbogenerator at the moment. Centrifugal pumps in safety related applications are normally on standby with the main discharge valve closed while descaling pump installations require frequent flow reductions near zero capacities. Even for general services, pump overheating protection can not be overlooked. For a common operating procedure calls for closing the downstream valve of the main discharge line at a pump startup. Then the pump must run at shutoff at least for the period before the valve is allowed to open by the procedure.
A common solution to the pump overheating problem is to install a bypass recirculation line for returning a certain flow from the discharge side of the pump to the upstream reservior. The return line is generally equipped with an orifice or a series of orifices for reducing the discharge pressure and, at the same time, dissipating the hydraulic energy as heat. The recirculation can be either kept continuously open or controlled by an on-off valve such that the bypass opens only when pumping capacity is reduced substantially lower than the rated capacity. The continuous recirculation system is most simple to install and to operate and it is absolutely safe. However the system requires an oversized pump to provide the rated main flow plus the bypass flow. A more serious disadvantage is that it wastes unnecessary energy in the recirculating flow when pump cooling is not needed. It is very often that the annual power costs for maintaining the excessive recirculation may run higher than the installation costs of the pump itself. This energy wasting feature is most undesirable and intolerable in the practice of pump applications. Unfortunately, continuous recirculation system is still in use, for it is the only available practical solution for overheating protection in many pump installations, especially for small scale pumping systems.
The modern pump installations set trends to control the system flow with automatic control devices. In this case the bypass must be either continuously open, as previously mentioned, or controlled automatically in response to the system flow variation. An automatic bypass control system consists, in general, a flow sensor for determining the flow rate at the moment, a signal rely to the bypass valve, a bypass valve actuator, an external power source for driving the bypass valve and some other electronic and mechanical components. For instance, to control the bypass flow of a boiler feed pump of a power plant the control system further requires a back-up manual valve on the bypass line, a valve position indicator and an alarm system for warning the operator in case the bypass fails to open. The installation and maintenance of this system are extremly expensive and it is unpractical for most pump applications.
Even with the burden of the high costs of the automatic control, the problem is not solved in a satisfactory manner. On one hand the bypass control system has potential hunting problem near the flow setpoint and on the other hand the main system flow experiences flow shocks as the bypass valve in motion. It is a common practice to increase the flow setpoint of the bypass control to that of twice of the minimum safe flow rate in order to avoid hunting. This will however further aggravate the flow shock problem of the system flow. In addition, it wastes energy for maintaining the extra recirculating flow. Multiple bypass system is sometimes used for mitigating the system flow shocks. This means even higher initial and maintenance costs and more complicated operation.
Many engineers and pump designers have continuously searched for better solutions to the problem of pump overheating protection. Useful improvements have been made in the areas of bypass design, orifice selection, component design for bypass valves and control system design. However, as mentioned above, no ideal solution is yet available for pump operation in the low flow range. It has been always my belief that the pump overheating problem can and should be solved from the source of the problem, namely the pump design. In the present disclosure I shall disclose an invention relates to a centrifugal pump that spontaneously responses to flow reduction to generate circulating cooling flow that protects the pump from overheating at low capacities. As a result, no external bypass control is needed and no wasteful energy is consumed, in accordance with the present invention.